Ultrasonic sensors have application in the characterisation of high frequency ultrasonic transducers and transducer arrays. This characterisation takes the form of output power measurements and beam profile measurements which determine the spatial distribution of output power. Such measurements also allow the calculation of other required parameters of ultrasonic devices. These measurements are particularly important in relation to ultrasonic devices for medical applications since the total output power and beam profiles must be accurately measured to ensure the safety of patients.
Hitherto the sensing of ultrasound has commonly been based on the use of a piezoelectric active element, such as a polyvinylidene-fluoride film. When the ultrasonic power is very high, this material can lose sensitivity or even become damaged. Additionally, the currently available size of these active elements is too large to resolve very narrow ultrasonic beams generated by highly focusing transducers. For these reasons, conventional hydrophones are not very satisfactory for characterising medical ultrasound that is of high power and highly focused.
A fibre optic ultrasonic sensor based on the use of a single-mode optic fibre has been proposed to alleviate some of these disadvantages. These devices operate on the principle that when an ultrasonic wave in the megahertz range is incident normally upon a single-mode fibre, the fibre becomes anisotropic, and consequently, the polarisation of the light at the output end of the fibre is modulated by the ultrasonic pressure along the fibre and thus varies at the ultrasonic frequency. The difficulty with sensors based on a single-mode fibre is that means must be provided to ensure that the light entering the region of interaction is circularly polarised, and that the optimum bias phase is also maintained at the same time. In practice the latter requires massive feedback control on optical components at both the input and output ends of the fibre to avoid signal fading due to environment disturbances to the fibre. This requirement has limited the application of ultrasonic sensors based on a single mode fibre.